Bideauxite is a new mineral found in specimens from the Mammoth-St. Anthony mine, Pinal County, Arizona. The composition is Pb2AgCl3(F,OH)2 with Pb, Ag, Cl, F by duplicate wet analyses. Z = 16, G. calc. 6·256, meas. 6·274 (at 24·2 °C). Crystals are isometric hexoctahedral with a 14·132 Å. Space group Fd3m. Crystals show a, o, d, m {113}, μ {114}, n {112}, φ {116}, and {029} in decreasing order of importance. Colourless, transparent with white streak. Isotropic with nD 2·192. le, almost sectile, withc onchoidal fracture. H = 3. Turns pale lavender in strong light. Low M.P. B.P. Decomposes in acids and warm NH4OH. Bideauxite is an oxide zone mineral found with leadhillite, matlockite, and anglesite. May alter to cerussite.